The present invention relates to a proximity sensor, which is a device which reacts to the presence of an object within a certain spatial volume, without direct physical contact between the proximity sensor and the object. The basic components of the proximity sensor are an energy source and a sensor. Interaction between the object and the energy causes a change in some aspect of the sensor output (amplitude, frequency, etc.) from which the presence of the object is inferred.
Among the applications for a proximity sensor are the fields of instrusion detection, the counting of parts produced by a production tool, and the protection of a machine operator from the hazardous area of a machine. Increasingly severe requirements, due to expanding technology and more alien environments, have necessitated research to improve performance parameters of present proximity sensors, as well as the development of new approaches.
An area of particular recent concern has been the need for a device as a guard or shutoff control for machine tools. The purpose of the device would be to deactuate a machine should the operator insert his hands into an area which is dangerous. For example, such an apparatus is needed for a punch, forging, or stamping press where it is desirable to cause the press to shut down at once and discontinue operation should the operator's hands be in the press area during operation.
A number of safety devices for machine tools have been proposed recently. Among the types of safety devices proposed are the light curtain type, the capacitance type, and the physical barrier type.
The light curtain type of safety device utilizes a light beam or beams which form a curtain of light. This may be achieved by the sequential operation of a plurality of light sources, as shown in U.S. Pat. No. 3,746,863, or by the scanning of a single light beam and the reflection of the scanned beam between a pair of spaced reflectors. When the light beam is interrupted so that it does not reach a detector, the machine tool is deactuated. One problem with these systems is that they are relatively complex. In the case of the light curtain of U.S. Pat. No. 3,746,863, a large number of light sources and detectors are required as well as circuitry for the sequential operation of these sources and detectors. In the case of the scanning light beam, only a single light source and detector are used, but moving parts, including a scanning mirror, are required.
The capacitance type of safety device uses an antenna near the dangerous area. The presence of an object near the antenna causes a change of the loading on the antenna which is then sensed. One problem with this type of sensor is that it will sense movement three feet or more away from the antenna, i.e. the space which it monitors is not well defined.
The physical barrier type of safety device utilizes a screen or plate which is interposed between the operator and the dangerous area. The machine will operate only when the plate is in position. In some applications, the operator must move the plate in order to load the machine and then replace the plate in order to start the machine. This causes additional operations to be performed by the operator, thereby reducing productivity. In other applications the physical barrier is moved automatically, but this is still a time consuming motion. In both cases the physical barrier obstructs the operator's view of the die.